Steel suitable for use as rolling elements

ABSTRACT

Steel suitable for use as rolling elements, such as bearings and gears, and a method for producing the steel are disclosed. The surface layer of the steel, which is directly responsible for the rolling-fatigue strength of the steel, is composed of a structure comprising quasi-carbide dispersed in the martensite matrix (hereinafter referred to as &#39;&#39;&#39;&#39;quasi-carbide structure&#39;&#39;&#39;&#39;).

United States Patent Kinoshi et al.

[ Dec. 30, 1975 STEEL SUITABLE FOR USE AS ROLLING ELEMENTS Inventors: Masao Kinoshi, Fujisawa;

Kyozarburo Furumura, Chigasaki, both of Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Aug. 27, 1973 Appl. No.: 391,597

Related U.S. Application Data Continuation-impart of Ser. No. 297,644, Oct. 16, 1972, abandoned.

U.S. Cl. 148/143; 148/165; 148/315;

148/36; 148/39; 148/144 Int. C1. C21D 1/18 Field of Search 148/143, 144, 36, 31.5,

[56] References Cited UNITED STATES PATENTS 3,595,711 7/1971 Faunce et al. 148/144 3,663,314 5/1972 Monma et a1 148/144 Primary ExaminerR. Dean Attorney, Agent, or FirmToren, McGeady and Stanger [57] ABSTRACT Steel suitable for use as rolling elements, such as bearings and gears, and a method for producing the steel are disclosed. The surface layer of the steel, which is directly responsible for the rolling-fatigue strength of the steel, is composed of a structure comprising quasicarbide dispersed in the martensite matrix (hereinafter referred to as quasi-carbide structure").

5 Claims, 28 Drawing Figures Retained Austenite US. Patent Dec.30, 1975 Sheet10f17 3,929,523

FIG. I

a i CI1 After Annealing C/2After Rapid Heating and Quenching 1 {C r/ -1After Annealing 6 Crl- 2 After Rapid Heating I C.Cr(lo) in Carbide and Quench'ng or Matrix II 4 I I Solid Dissolutinn 2 Limit of C in Austenite 2 f l MIC R 0 NS OI Outer Shell Outer Shell Matrix (Martensite) Retained Austenite US. Patent Dec.30, 1975 Sheet2of 17 3,929,523

U.S. Patent Dec. 30, 1975 Sheet3 of 17 3,929,523

x5000 5% picml etched F IG.4

Quenchinq Temperatur 950C x5000 Average Heating Rai'e 5C/sec- FIGS U.S. Patent Dec.30, 1975 Sheet4of 17 3,929,523

X10000 5% picral efched etched lqooo 5% p cml (c =1.o5% FIG.7

US. Patent Dec.30, 1975 Sheet5 of 17 3,929,523

icral efched x5000 5%picral el'ched US. Patent Dec.30, 1975 Sheet6of17 3,929,523

OP x 1900 FIGJOQ FlCllOb U.S. Patent Dec.30, 1975 Sheet7of 17 3,929,523

Cr xiQQo FIGJ \&

C x i900 US. Patent Dec. 30, 1975 Sheet80f 17 3,929,523

FIG. I2

Max.Cand Crl CI /o in Cementite in Quasi-Carbide O (2% in Cementite Cr 5 o O C o I 1 I I 000 l I00 I 200 Maximum Heating Temperature (C) Average Heatig Rate 50*120 '9ec above 850C Surface of Carburized Layer:C/:1.36/

US. Patent Dec.30, 1975 Sheet9of 17 3,929,523

l Cementite Diffused Layer Higher Temperature 3 Quasi-Carbide (ealier stage) Convexed Quasi-Carbide (middle stage) Quasi-Carbide (final: stage) two quasi-carbides combined together Quasi-Carbide(final stage) Martnsite in the-matrix to grow coarse Retained Austenite US. Patent Dec. 30, 1975 Sheet 10 of 17 3,929,523

FIG. l4

t O tzHe-uting Time Cc C Cc: (2% in Cementite (b)t tl Cc Microns US. Patent Dec. 30, 1975 Sheet1lof17 3,929,523

Amount of Cementite Temperulure FIG. l6

T| C Rapid Cooling A B U t2 t Sec) 850C Gpld Coollng A B t1 t2 t( Sec) US. Patent Dec. 30, 1975 Sheet 12 of 17 3,929,523

FIG. I?

0 Quasi-Carbide Structure 0 Average Life Ratio as Compurd With Material Quenched 0 in cm Electric 0 o Furnace 5 I i l 1 I000 I I00 I200 Max. Heating TemperutureCC) FIG. l8

Quasi-Carbide Structure 5 -60 0 Average Life Ratio as p d t Residual Stress Material Quenched in a an Electric Furnace Life Ratio Residual Stress i I i000 l I00 I200 Max.Hec1ting Temperature (C) US. Patent Dec. 30, 1975 Sheet 13 of 17 3,929,523

Average Life Ratio as Compurd With Material Quenched in 0 cm Electric Furnace I N00 I200 Mcx.Hec1ting Temperature (C) FIG. 20'

Depth from Surface (mm) US. Patent Dec.30, 1975 Sheet 14 of 17 3,929,523

FIG. 2|

* Quasi-Carbide Stracture 0 Average Life Ratio as Compard With 0 Material Quenched in an Electric Furnace IOOO IIOO I200 Max. Heating Temperature (C) Average Life 0 Ratio as Compard With Material Quenched in an Electric Furnace b Quasi-Carbide Stracture 5 o O o o I I0 IOO IOOO Average Heating R-1te( /ec) US. Patent Dec. 30, 1975 Average Life Ratio as Compurd With Material Quenched in cm Electric Furnace Sheet 15 of 17 FIG. 23

Acciculur Martensite Quasi-Carbide Structure +Remined Austenite (SSI'HQH Amount of uesi-Curbide r and Cementite Quesi-Carbide Ol t Preheuting Temperature ("(1) 2O Quesi-Curbide Vol It:

US. Patent Dec.30, 1975 Sheet 16 of 17 3,929,523

FIG. 24

Quasi-Carbide 5 Structure Average Life Ratio as Compord With Material Quenched in an Electric Furnace 900 I000 II00 I200 Max. Heating Temperature (C) 0.6 C.N/a

O I I I Depth from Surface (mm) STEEL SUITABLE FOR USE AS ROLLING ELEMENTS CROSS-REFERENCE TO PRIOR APPLICATION:

This is a continuation-in-part of Ser. No. 297,644 filed Oct. 16, 1972 and now abandoned.

FIELD OF INVENTION The present invention relates to steel for rolling elements, such as bearings and gears, exhibiting improved rolling fatigue strength characteristics.

BACKGROUND INFORMATION For improving the rolling fatigue strength of steel, various techniques have been developed. In the field of steelmaking processes proper, vacuum degassing and vacuum remelting have been proposed for this purpose. In the field of heat treatments, mar-stressing is oftentimes performed, while in the field of steel-working specific fiber orientation by hot-working and aus-forming has been attempted.

However, the improvement in the rolling-fatigue life obtained by these conventional procedures is very limited and these procedures cause various problems, such as increased production cost, low productivity and limitation of steel grades.

SUMMARY OF INVENTION The present invention remarkably improves the rolling-fatigue life by combining a particular melting method with a particular working method, while at the same time enhancing the productivity. The invention is applicable both to throughhardened steel and casehardened steel.

The present invention provides a novel steel for rolling elements and a method for producing the same. Briefly, the steel is characterized in that the steel surface layer which is directly responsible for the rollingfatigue strength has a structure which is dispersed with quasi-carbide (a granulated substance as hereinafter defined) in the martensite matrix (hereinafter called a quasi-carbide structure).

The present invention will be explained by referring to the attached drawings.

FIG. 1 is a graph showing schematically the structure and chemical composition of the quasi-carbide,

FIG. 2 shows results of line-scanning profile by X-ray microanalysis of C and Cr contents in the cementite and their back scattered electron images,

FIG. 3 shows results of line scanning profile by X-ray microanalysis of C and Cr contents in the quasi-carbide and their back scattered electron images;

FIG. 4 is an electron-microscope photograph showing a bearing steel as annealed;

FIG. 5 is an electron-microscope photograph showing a quenched structure of a bearing steel (JIS G 4805 SUJ2);

FIG. 6 is an electron-microscope photograph showing the quasi-carbide structure of the bearing steel (JIS G 4805 SUJ2) as rapidly heated and quenched;

FIG. 7 is an electron-microscope photograph showing the quasi-carbide structure of CrMo steel as rapidly heated and quenched;

FIGS. 8 and 9 are electron-microscope photographs of the quasi-carbide structures of the bearing steels SUJ3 and SUJ2 (JIS G 4805), respectively;

FIG. 10 shows the characteristic X-ray image of the cementite by an Xray microanalyser;

FIG. 11 shows the characteristic X-ray image structure of the quasi-carbide by an X-ray microanalyser;

FIG. 12 shows the relation between the maximum heating temperature and the C and Cr contents in the quasi-carbide;

FIG. 13 explains the destruction process of the cementite in case of rapid heating;

FIG. 14 explains the dissolution process of the cementite in the austenite in case of an ordinary heating;

FIG. 15 explains the dissolution amount of the cementite into the austenite in case of an ordinary heating and rapid heating;

FIG. 16 explains the temperature-time curve for a rapid heating;

FIG. 17 shows the relation between the maximum heating temperature and the rolling-fatigue life of the bearing steel (JIS G 4805 SUJ2) rapidly heated and quenched;

FIG. 18 shows the relation between the maximum heating temperature and the rolling-fatigue life of the bearing steel (JIS G 4805 SUJ2) as quenched by high frequency rapid induction heating;

FIG. 19 shows the relation between the maximum heating temperature and the rolling-fatigue life of a carburized Cr-Mo steel as quenched by high frequency rapid induction heating;

FIG. 20 is a distribution curve of carbon contents of the carburized layer of Cr-Mo steel;

FIG. 21 shows the relation between the maximum heating temperature and the rolling-fatigue life of a carburized NiCrMo steel as quenched by high frequency rapid induction heating;

FIG. 22 shows the relation between the average heating rate from room temperature and the rolling fatigue life;

FIG. 23 shows the relation between the pre-heating temperature in a quenching by rapid heating and the rolling-fatigue life as well as the amount of the quasicarbide;

FIG. 24 shows the relation between the maximum heating temperature and the rolling-fatigue life of a carbonitrized CrMo steel as quenched by high frequency rapid induction heating;

FIG. 25 shows curves of carbon and nitrogen concentrations in the carbonitrized layer of the Cr-Mo steel; and

FIG. 26 shows the relation between the sliding distance and the wear amount in the wear tests made on the bearing steel (JIS G 4805 SUJ2) as ordinarily quenched and the same as quenched by high-frequency induction heating.

The term quasi-carbide structure used in the present invention has the following meanings:

The quasi-carbide structure is a structure in which quasi-carbide as defined hereinafter is chiefly dispersed in the martensite matrix, and satisfies the following conditions:

1. Volume of the quasi-carbide is 15 2. Size of the quasi-carbide (regarded as spherical) is about 0.2 10 y. in diameter.

The quasi-carbide is produced not by slowly dissolving the granular cementite above the A temperature into austenite, but rather by converting the granular cementite by one step into a new compound which is similar to but, in fact, different from the cementite.

The characteristics of the quasi-carbide are:

1. It is almost spherical conceptionally and is composed of a core portion and an outer shell portion. As shown by the curve 2 in FIG. 1 and the linear analysis of C and Cr by X-ray microanalyser in FIG. 3, the quasi-carbide has lower C and Cr contents than the C and Cr contents of the cementite indicated by the X-ray microanalysis in FIG. 1 and FIG. 2, and yet has an increasing C content toward the core portion and a decreasing C content toward the outer shell portion. This is in contrast to the uniform composition of the cementite proper. According to the measurements by the X-ray microanalyse'r, the C content in the core portion is about 2 5% which is not characteristic of a cementite nor a retained austenite produced by a conventional quenching method. Thus, this is considered to be a completely new structure.

The outer shell portion of the quasi-carbide is composed of retained austenite, but the carbon dissolved therein decreases continuously toward the matrix.

Alloying elements, such as Cr, Mn and Mo, which have strong affinity to carbon are also diffused, but not as much as carbon, and take a similar distribution pattern than that of carbon.

The carbon content in the boundary zone between the core portion and the outer shell portion is estimated to be around 1.7% in view of the fact that the outer shell portion is composed of retained austenite and the dissolution limit of carbon in austenite is about 1.7%.

The differences in the composition between the cementite proper and the quasi-carbide are as mentioned above. Thus, when observed by an electron micro- I scope, the differences are clearly seen by comparing degree varies slightly depending on the heating conditions and the steel grades.

In the quasi-carbides shown in FIG. 6 and FIG. 8, the boundary with the matrix is obscure and a linear etching pattern produced in the grain is observed in the quasi-carbide formed from a large cementite. It is one of the characteristics of the quasi-carbide that a number of the linear etching patterns are observed when a quasi-carbide is formed from a large cementite. FIG. 7 shows a microstructure of case-hardening steel which has been carburized and annealed and then converted into the quasi-carbide structure. The center portion having a relatively clear boundary like a cementite, corresponds to the core portion as mentioned before. In this case, the center portion is particularly hard to corrode because the concentration of Cr and M0 in the center portion is high and also because the discrepancy between the diffusion zone of C and the diffusion zone of Cr and M0 is remarkable.

FIG. 9 shows a typical quasi-carbide which was pro duced by heating the steel composition SUJ2 (JIS G 4805, similar to AISI 52100) to lO80C at an average heating rate of about 400C/sec.

Observations showing the above-mentioned characteristics of the quasi-carbide are shown in FIGS. 10a, b and FIGS. 11a, b. FIG. 10 is a characteristic X-ray image (photographed by an X-ray microanalyser) of the primary cementite in the steel C of Table 1, which was carburized and quenched, and FIG. 10a shows the image of CrKa, and FIG. 10b shows the image ofCKa. When FIGS. 10a and b are superimposed on each other, the Cr-rich region and the C-rich region coincide. This indicates that the cementite contains a large amount of Cr and C.

FIG. 11 shows the characteristic X-ray image of a quasi-carbide structure converted from the cementite shown in FIG. 10 (C Q Table 2). When the image of CrKa of FIG. 11a and the image of CKa of FIG. 1 1b are superimposed, it becomes clear that Cr is contained in a large amount in the quasi-carbide, but the C content has been remarkably reduced.

2. The quasi-carbide takes various modes depending upon heating conditions.

Referring to the carbon content (maximum) in the core portion of the quasi-carbide structure only, the carbon content decreases as the heating temperature increases, as shown in FIG. 12, but to a much lesser degree as compared with the chromium content, for example. This indicates that the diffusion of chromium is remarkably slow as compared with that of C.

Together with the above structural changes, the microstructures viewed by an electron microscope also change from 3 to 7 in FIG. 13 as the heating temperature increases. In this case the effects of the average heating rate are that the change of the quasi-carbide shifts to the higher temperature side as the heating rate is increased; 3 to 7 of FIG. 13 show the quasi-carbide. FIG. 7 corresponds to 3 of FIG. 13, FIG. 6 corresponds to 3 of FIG. 13, FIg. 6 corresponds to 4 of FIG. 13, FIG. 8 corresponds to 4 and 5 of FIG. 13, and FIG. 9 corresponds to 5 of FIG. 13.

Actually, individual cementites in the same steel material vary from each other and it is not possible that all of the cementites take the same mode; thus, quasicarbides coexist in stepwise manner. For example, a structure composed mainly of the quasi-carbide of 3 and 4 together with a small amount of the carbides of 2 and 6 may be obtained. The forming mechanism of the quasi-carbide is explained in the following:

In respect of a steel containing more carbon than corresponds to the eutectoid, and when the heating temperature exceeds the Ac, transformation point and the matrix has been transformed into austenite, the cementite is dissolved into the austenite as the temperature further increases. In this case, if the heating rate is slow, the cementite becomes smaller in an equilibrium state maintaining the constant composition of (a) (b) (c) in FIG. 14. On the other hand, if the temperature is raised suddenly, the free energy of the cementite becomes so large that it is no longer possible to reduce the free energy suddenly by the equilibrium diffusion as shown in FIG. 14, and thus the cementite phase disappears at once as if concentration of carbon did not exist as the cementite phase, but only an irregular distribution of concentration existed in one phase as shown in FIG. 1.

Regarding the change in the cementite amount, two different forms are taken depending on the heating rate bordering on a certain heating rate (dT/dt)c as shown in FIG. 15.

The curve 1 in FIG. 15 is commonly seen in the conventional austenitizing treatment, and the curve 2 is seen in case of rapid high temperature heating.

T represents a temperature at which the quasi-carbide starts to form, which temperature is higher than the Acm point in a hyper-eutectoid steel and higher than the A point in a hypo-eutectoid steel. 

1. Steel suitable for use in rolling elements and exhibiting improved rolling-fatigue strength, the surface layer of said steel consisting essentially of the following chemical composition; C, 0.65 - 1.4%; Mn 0.04 - 1.5%, Si 0.04 - 2.0% and one member selected from the group consisting of 0.06 - 0.6% Mo; 0.30 -2.5% Cr; and a combination of 0.06 - 2.5% Cr and 0.06 - 0.6% Mo wherein Cr+Mo > or = 0.2%; with the balance being iron and unavoidable impurities, said surface layer having a martensite matrix and having dispersed within said matrix 15 - 80 volume % of quasi-carbide particles composed of a core portion and an outer shell portion, said core portion having a carbon content of about 2 - 5%, said outer portion being composed of retained austenite having a carbon content which decreases continuously toward the outer shell portion and which further possesses lower carbon and chrome contents than cementite said quasi-carbide being in the form of particles of about 0.2 - 10 Mu in diameter.
 2. A METHOD FOR PRODUCTING STEEL FOR ROLLING ELEMENTS WHICH COMPRISES HEATING A STEEL MATERIAL TO A PREDETERMINED TEMPERATURE BETWEEN ABOUT 1000* AND 1250*C AT AN AVERAGE HEATING RATE OF MORE THAN 10*C SEC. ABOVE 850*C AND QUENCHING THE THUS HEATED STEEL MATERIAL TOABTAIN A STEEL WHEREIN THE SURFACE LAYER HAS A MARTENSITE STRUCTURE WITH 15 TO 80 VOLUME % OF QUASI-CARBIDE AND HAVING A CARBON CONTENT WHICH DECREASES CONSTRUCTURE, SAID PARTICLES BEING IN THE SIZE RANGE FROMABOUT 0.2 TO 10 MICRONS ANND HAVING A CORE PORTION AND AN OUTER SHELL PORTION,SAID CORE PORTION HAVING A CARBON CONTENT OF ABOUT 2 TO 5% AND SAID OUTER PORTION BEING COMPOSEDOF RETAINED AUSTENITE AND HAVING A CARBON CONTENT WHICH DECREASES CONTINUOUSLY TOWARD THE OUTER SHELL PORTIONN PORTION WHICH FURTHER PROCESSES LOWER CARBON AND CHROME CONTENTS THAN CEMENTITE, THE SURFACE LAYER OF SAID STEEL STARTING MATERIAL CONSISTING ESSENTIALLY OF THE FOLLOWING CHEMICAL COMPOSITION; C, 0.65 - 1.4%; MN, 0.04 - 1.5%; SI, 0.04 - 2.0%, ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF 0.06 0.6% MO, 0.30 - 2.5% CR, AND A COMBINATION OF0.06 2.5% CR AND 0.06 - 0.6% MO WHEREIN CR + MO 0.2%, THE BALANCE BEING IRON CONTAINING 5 - 40 VOLUME % OF GRANULAR SURFACE LAYER CONTAINING 5 - 40 COLUME % OF GRANULAR CARBIDE COMPOSED MAINLY OF CEMENTITE AT A TEMPERATURE BELOW 850*C.
 3. A method for producing steel for rolling elements which comprises heating a steel material to a predetermined temperature between about 1000* and 1250* at an average heating rate of more than 10*C/sec. above 850*C and quenching the thus heated steel material to obtain a steel wherein the surface layer has a martensite structure with 15 to 80 volume % of quasi-carbide particles distributed through the martensite structure, said particles being in the size range from about 0.2 to 10 microns and having a core portion and an outer shell portion, said core portion having a carbon content of about 2 to 5% and said outer portion being composed of retained austenite and having a carbon content which decreases continuously toward the outer shell portion and which further possesses lower carbon and chrome contents than cementite, the surface layer of said steel starting material consisting essentially of the following chemical composition; C, 0.65 - 1.4%; Mn, 0.04 - 1.5%, Si, 0.04 - 2.0%, one member selected from the group consisting of 0.06 - 0.6% Mo; 0.30 -2.5% Cr, and a combination of 0.06 - 2.5% Cr and 0.06 - 0.6% Mo wherein Cr + Mo > or = 0.2%, and at least one element selected from the group consisting of Ni, 0.25 - 5.0% V, 0.03 - 0.2% W, 0.10 - 0.8%, and B, 0.001 - 0.01%, the balance being iron and unavoidable impurities, said surface layer containing 5 - 40 volume % of granular carbide composed mainly of cementite at a temperature below 850*C.
 4. The method of claim 3 wherein the average heating rate is more than 20*C/sec.
 5. Method according to claim 2 in which the steel material is rapidly heated from room temperature to a temperature between about 100* and 1250*C with an average heating rate of more than 25*C/sec. 